1. Field of the Invention
The present invention relates to comminuting machines in which the feedstock is delivered to a rotary fragmenting machine by gravitational force and, more particularly, to the devices and methods used by gravity-fed grinding machines to improve grinding efficacy.
2. Description of the Related Art
For many applications and work environments, a comminuting machine in which the feedstock enters the fragmentation zone through a gravity chute offers many advantages over machines with other feed delivery and feed rate regulation methods. In a typical machine of this kind, the feedstock is deposited into a gravity chute by a conveyor, auger, or other appropriate device, or by the operating force of another comminuting device. At the bottom of the gravity chute, the feedstock encounters a rotationally powered fragmentation device, commercially known as a rotor or hammer mill, with peripherally mounted comminuting instruments, commonly referred to as teeth, hammers, cutters, and other names suggestive of their function, extending therefrom. These teeth revolve about an axis generally perpendicular to the flow of feed materials at speeds typically exceeding 1000 rpm's, though lower speeds are also found on such devices. When an object enters the radial path of a rotor tooth, it is carried into a plate or bar that is fixed in place and generally labeled an anvil. After the initial striking of the feed material by a rotor tooth, the anvil, located a short distance beyond the outer circumferential path of the teeth, facilitates a second stage of the fragmentation process, as the feed material is subjected to great shearing and pulverizing forces between the radially traveling tooth and the anvil. After the material passes beyond the anvil, it circulates between the teeth and a sizing screen, an apparatus concentrically surrounding a portion of the rotor with apertures roughly the size of the desired finished product. Frangible objects continue to be broken down between the teeth and screening apparatus until they are small enough to pass through these apertures.
The relative simplicity of such a machine, as compared to machines commercially known as horizontal grinders and tub grinders, is an advantage, as well as a disadvantage in certain ways, as described below. Because the feed material enters the fragmentation zone by gravitational force, the power-driven components of the feedstock support and delivery system are not subjected to impact stress from the comminuting process, as they are in horizontal grinders and tub grinders. The drive and driven components of the feedstock conveyance systems for gravity-fed comminuting machines may therefore require less material strength and less maintenance. Gravity feed comminuting machines are particularly suited for processing feedstocks with small particle sizes, including pre-ground wood, bark, asphalt shingles, paper, agricultural waste, and municipal solid waste. Collection of fine debris between components of comminuting machines, a condition commonly present with the aforementioned types of feedstocks, often results in combustion and/or wear. Because the gravity chute separates the power driven feed components from the turbulence of the fragmentation zone, a gravity fed comminuting machine typically is not subject to the same levels of feedstock spillage and collection between feed components as horizontal grinders and tub grinders.
However, such machines generally experience a higher degree of feed rate irregularity than horizontal grinders and tub grinders. Typically, once the feedstock enters the gravity chute, its flow is regulated only by the size of the aperture above the rotor. The feedstock may enter the comminuting zone too quickly, reducing rotor speed, or in more extreme circumstances, stalling the rotor. Manufacturers have attempted to solve this problem by providing adjustable feed openings, such as described in U.S. Pat. No. 5,657,933. Yet this regulation method does not affect another common feed rate problem, the formation of chute obstructions. Friction, cohesion, and adhesion often cause the material flow to cease, slow down, or become irregular, resulting in operational energy inefficiency.
Without a power-driven component forcing the feed material into the fragmentation zone, feed materials may not effectively enter or remain inside the fragmentation zone. The air displaced by the movement of the rotor teeth may push light materials, such as paper and bark, away from the fragmentation zone. The rotor may eject objects from the fragmentation zone upon initial contact. If an object passes over the sizing apertures and travels through the circumferential path of the rotor teeth until it reaches the feed opening, it can escape the comminuting chamber at a high velocity. This feedstock ejection can create several problems. On machines lacking sufficient enclosure, object ejection can pose a serious threat to the persons in the vicinity of the machine, as well as necessitating additional labor to clean the area and handle the unprocessed material.
Feedstock may encounter the rotor teeth several times before passing through a sizing aperture as a result of repeated ejection up into the feed opening and subsequent descent into the comminuting zone. Each encounter with the comminuting zone may result in the feedstock fragmenting into smaller pieces. In these situations, machine operators may not experience effective control over particle size and texture. Repeated and excessive contact between the rotor teeth and individual pieces of feed material also reduces production efficiency and increases component wear in proportion to output.
In some gravity-fed comminuting machines, this material ejection and reintroduction to the comminuting zone problem is addressed with long feed chutes that increase the momentum and/or gravitational compression of the feed material entering the fragmentation zone. The height of a long feed chute may increase the cost of a machine and the equipment operating in coordination with it and decrease the machine's application versatility. Gravity fed comminuting machines, commercially known as wood hogs, are often installed in systems customized to specific environments and applications to operate in tandem with other machines. The size and shape of the gravity chute can significantly limit the feeding method options in comparison to horizontal grinders and tub grinders.
These problems, among others, limit the use of such known gravity-fed comminuting machines primarily to static operation within automated systems, such as commonly found in municipal solid waste processing plants, sawmills, and other facilities with constant flows of heterogeneous feedstocks. Machines of this kind are typically selected for their low investment costs and supervision requirements resulting from their relative simplicity. An effective solution to the problems inherent to gravity-fed comminuting machines must alleviate these problems without significantly increasing the cost or complexity of the basic design, a primary objective of the present invention.
The present invention addresses, inter alia, these listed deficiencies in the known devices.